/Engine/Source/Runtime/MessagingCommon/Public/MessageEndpointBuilder.h
WithHandler
template<typename MessageType, typename HandlerType> [FMessageEndpointBuilder](API\Runtime\MessagingCommon\FMessageEndpointBuilder) & Handling ( HandlerType * Handler, typename [TRawMessageHandler](API\Runtime\MessagingCommon\TRawMessageHandler)< MessageType, HandlerType >::FuncType HandlerFunc )